Utility pattern is a concept distinct from embroidery pattern sewing and hereinafter used to include all kinds of patterns deriving from a zigzag stitch pattern. Further, a pattern includes a pattern formed by stitches and may occasionally be referred to as a stitch pattern. Also, left, right front and rear denotes a relative direction having their basis on the forward direction relative to the user which is defined as the front direction.
Conventionally, a machine-driven pattern generating mechanism having a plurality of utility-pattern needle swing cams and a plurality of feed cams is arranged to bring a contact in abutment with the needle swing cam and the feed cam of the intended pattern by manually rotating a pattern selection dial. Thus, the machine-driven pattern generating mechanism generates stitch patterns by a combination of needle swing operation and cloth feed operation rendered by mechanical operation of each relevant component to form the desired utility pattern on the workpiece cloth. Nowadays, various types of pattern generating mechanisms have been suggested which are capable of button-hole sewing in addition to forming ordinary utility patterns.
For example, Japanese patent document 1, more specifically, JP-S62-014316 B (pages 4 to 5 and FIGS. 1 and 3) describes a button-hole forming unit of a zigzag sewing machine having a utility-pattern pattern cam, automatic feed cam and feed control cam for button-holes, and a worm wheel which are formed integrally on a cam shaft of a sewing machine arm. When button hole sewing is selected by rotating the pattern selection dial, feed amount of a feed regulator is controlled by a swinging movement of a feed contact and a needle bar is swung by a swinging movement of a swing-width contact so as to form button hole stitches constituted by a set of side-stitch portions in the left and right and bar-tack portions in the front and rear.
The button hole forming unit of the zigzag sewing machine described in patent document 1 has the utility-pattern pattern cam and the button-hole cam provided integrally on a cam shaft so as to be adjacent one another. This leads to increase in the axial dimension of the cam which in turn requires larger axial space and consequently enlarging the sewing machine.
Normally, a pattern cam for utility patterns employ 18-division pattern cam, and small-size household sewing machines employ 12-division pattern cams to achieve compactness at the expense of number of sewable patterns. However, fully automatic button hole stitching requires 6 stitches for each front and rear bar-tack portion and 3 stitches for each left and right side-stitch portions amounting to a total of 18 stitches, thus in need of a 18-division button-hole cam.
Also, as described in patent document 1, since the pattern cam and the button-hole cam are rotated in synchronism, the pattern cam for utility patterns needs to bear 18-divisions for adjustment with 18 stitches of the fully automatic button-hole stitching.
Other related art such as U.S. Pat. No. 4,428,311 (FIG. 6 and the corresponding description) describes a pattern selection mechanism of a sewing machine having a pattern cam shaft 21 provided in an arm so as to be perpendicular to a main shaft 20 and an operation shaft 30 parallel to the pattern cam shaft 21. The pattern cam shaft 21 has a first cam group 24 (for example pattern cam group) provided thereto while the operation shaft 30 has a second cam group 32 and a selection cam 33 and the pattern cam shaft 21 has a gear 26 provided thereto. The operation shaft 30 also has a gear 31 pivotally supported thereto.
While the gears 26 and 31 establish an engagement, the gear 26 is engaged with a worm wheel 19 which also establishes engagement with worm 23 secured to the main shaft 20. A first reduction mechanism composed of the worm 23 and the gear 26 has a reduction ratio of 1/6 and the reduction ratio of a second reduction mechanism composed of the first reduction mechanism and the gears 26 and 31, amounts to the total reduction ratio of 1/12.
The structure disclosed in patent document 2 enables the cam shaft and the operation shaft to be shortened, allowing size reduction of the sewing machine. Moreover, since the rotation times of the cam shaft and the operation shaft can be modified, if a button-hole cam is applied to the second cam group, the number of division of the pattern shaft provided the cam shaft and the number of division of the button-hole cam provided on the operation shaft may vary depending on the reduction ratio.
The pattern selection mechanism of the sewing machine described in patent document 2 is arranged to modify the reduction ratio between the pattern cam shaft 21 and the gear 26 on the operation shaft. Thus, the number of stitches (number of divisions) for ordinary utility patterns and the number of stitches (number of divisions) for button-hole stitches can be modified accordingly. However, allowing such modification in reduction ratio requires provision of the above described worm 23, worm wheel 19, and two gears 26 and 31, thereby increasing the number of parts, and moreover enlarging the drive mechanism composed of a cam shaft and an operation shaft, consequently leading to increase in manufacturing cost.
Furthermore, sequential engagement of the worm 23 and worm wheel 19 and the two gears 26, 31 results in increase in number of engagement, which in turn leads to lower transmission efficiency of drive force. Moreover, rattling caused by backlash impairs precision of gear rotation and may cause impairment in precision of rotational positioning of the pattern cam and the button-hole cam.